Learning Outcome
1.1 Identify and gather soil analysis tools, supplies and
materials
1.2 Identify engineering properties of soils
1.3 Analyse properties of soils
1.4 Prepare soil analysis report
Analyse Soil Properties
Introduction to learning outcome
Definition of key terms
Moisture Content—Moisture content is the amount of water present in a given soil mass.
PI index—Plasticity index is a measure of the range of water in which the given soil
remains in plastic state
Sieve Analysis—Sieve analysis is a procedure for classifying soils using standard sieves
to determine the particle size distribution.
1.1 Identify and gather soil analysis tools, supplies and materials (Sieve analysis, PI
index, Moisture content, CBR, Proctor, Triaxial test, Oedometer tests,
Casagrande, Cone penetrometer, Sand Replacement, California Bearing Ratio)
based on available resources and the tests to be conducted
The following explains the gathered soil analysis tools, supplies and materials based on
Tests to be conducted:
CBR Test- This refers to California Bearing Ratio which is a penetration test used
for classification of soil sub grade and base course materials, especially for
design of flexible pavements and was developed by California State Highway
Department in U.S.A.
It shows the ratio of force per unit area needed to penetrate a given mass of soil at
a rate of 1.25 mm/min to the corresponding penetration of a standard material.
Proctor Test- This is a compaction test that is carried out to determine the
optimum moisture content at which a given soil in question achieves maximum
dry density through air removal.
Triaxial Test- This test is done to measure the mechanical properties such as shear
strength of soil; hence, it is often known as triaxial shear strength. It is performed
by subjecting stress on a given soil sample in a way that the resultant stress in one
Direction is different in the perpendicular direction.
Oedometer Test—This test is done to determine consolidation, which is an
engineering property of soil. This is determined by measuring the vertical
displacement of the soil being tested when it is subjected to vertical loading
Casagrande Test: This is an atterberg limit test used to measure the liquid limit
of a given soil sample. The term “Casagrande” is used because the test apparatus was
designed by Arthur Casagrande.
Cone penetrometer- This is used in testing soil properties at the site because one
is able to map out the soil profiles and determine properties such as relative density
of soil.
Sand Replacement Test—This test is used to determine the dry density of soil at
the site. One needs to dig a hole first then proceeds to fill it with sand of a known
density, hence the term “sand replacement.” From this, one can determine the volume
of the hole.
1.2 Identify engineering properties of soils based on the soil classification
Engineering properties of soils are the characteristics that soils possess that make them
suitable for use in the engineering field. They range from one soil to another and in that
way soil has to be classified into different categories as shown below
a) AASHTO classification system
This system is based on the plastic characteristics and particle size of soil. The soil is
divided into seven categories and even further when considering the group index as shown
in the classification chart below:
Figure 184: AASHTO classification
Source: Bello, Afeez. (2013). Introductory Soil Mechanics I.
b) Unified soil classification system
This system classifies soil into three categories: Organic soils, coarse-grained and fine
grained soils. This is determined depending on how much soil passes standard sieve sizes. c) Grain size classification system
This system classifies soils based on the grain size only; hence, it is not very significant. The
soil can be classifying as either silt, sand or clay depending on the particle size.
d) Textural classification
This is an improved system from the grain size system because it considers the grain size
distribution as a percentage.
From the above classification systems, the following engineering properties can be
identified as shown:
Shear Strength—This refers to soil resistance to deformation when subjected
to shear stress. It is important in geotechnical engineering because it helps in
determining the bearing capacity of the foundation.
Consolidation—According to (Terzaghi & Peck, 2013), consolidation is
“any process which involves a decrease in water content of
saturated soil without replacement of water by air”.
Liquid Limit—This shows the amount of moisture present when a given soil
sample transitions from plastic state to liquid state such that the soil can flow.
This helps classify the soil in terms of how fine it is and it is important to an
engineer as they can calculate the allowable bearing capacity of foundation as
well as its settlement.
Dry Density – This, in simple terms is the density of soil when dry and is
calculated by subtracting the moisture content from the wet density. It is used
to give an idea of extent of compaction of soils whereby a high dry density
indicates an increased compaction.
Relative Density- This property is dimensionless and shows how dense a
cohesionless soil is, that is the difference between void ratios when loose and
in natural state, compared to its maximum density i.e. the difference in void
ratio when loose and dense state.
1.3 Analyse properties of soils based on the standard procedures
The following standard procedures are used in analyzing engineering properties of soils:
They are divided into two main categories:
i. In-situ Testing Procedures: This involves field tests carried out at site of proposed
location and may include but not limited to:
a) Standard Penetration Test
This test is done in accordance to IS -2131:1963. A bore hole is drilled to a
desired depth after which a standard split spoon sampler is driven into the soil
using a dropping hammer weighing 63.5kg it is dropped at a rate of 30 blows
per minute over a height of 0.75m. The aim of this test is to determine the
relative density of soil.
b) Sand Replacement Test.
This test aims at determining the dry density of the soil in question and it is
based on IS 2720 part 28. The in situ compacted soil is removed and replaced
with sand of known density to determine the volume of the hole after the weight
of sand needed to fill the hole is determined.
ii. Laboratory Tests: This cannot be done at site hence the soil sample is taken to the
laboratory for analysis. They include but not limited to:
a) Sieve Analysis
This test is used to determine the particle size distribution of soil using the
standard procedure outlined in ASTM C136 (AASHTO T-27). The sample soil
is passed through a number of standard sieves and the retained soil in each sieve
measured for analysis in determining the soil gradation.
b) Atterberg’s Limits Tests
This involves a number of tests that are carried out to determine the critical
water content in fine graded soils. The tests are carried out as outlined in
ASTM D4318 and include: liquid limit test, plastic limit test and shrinkage limit
test.
1.4 Prepare soil analysis report based on the results.
A soil analysis report is a report prepared by an engineer that shows the soil properties
of the area where an infrastructure is likely to be constructed showing whether the soil
conditions are suitable on not. It is important because it tells the viability of proposed
project before it is started.
The following is the procedure for preparing a soil analysis report:
i.
One is first required to know the client’s expectations on the proposed
project for instance the location where they want the project to be among
others.
ii.
The next step is carrying out both field and labouratory tests of the soil on
the chosen location to determine its engineering properties. The tests and
soil properties have been highlighted in 1.3 and 1.2 respectively.
iii.
The results from the tests are the analysed to determine if the soil has
suitable engineering properties thus a decision can be made whether the site
is safe for engineering works.
iv.
The Geotechnical engineer will then provide a detailed report of existing
conditions to the client as well as give recommendations where necessary
which helps in knowing if the proposed are safe and viable and what can be
done incase otherwise.
Conclusion
This learning outcome covered analysis of engineering soil properties in terms of what they
are, how they can be tested together with relevance in engineering field and how a soil
analysis report can be prepared.
19.3.2.3 Self-Assessment
Written Assessment
1. The following are labouratory tests in soil analysis except one:
a) Standard Proctor Test
b) Penetration test
c) Triaxial Test
d) Odometer Test
2. Soil analysis is important in engineering works. Which if the following is not an
engineering property of soil:Shear Strength
a) Plastic Limit
b) Consolidation
c) Water quality
3. What is an in-situ testing procedure? Give three examples of in-situ tests in soil
analysis
4. Elabourate 4 Atterbergs Limits
5. Discuss 4 engineering properties of soil
6. What is the difference between density of soil and relative density of soil?
7. Proctor test is used to determine the optimum moisture content of soil. Briefly explain
how the proctor test is done and its significance in the engineering field.